1. Field of the Invention
This invention relates to integrated circuit manufacture and more particularly to intrinsic gettering of a silicon substrate and processing steps used to maintain a denuded area and gate oxide quality upon the denuded area.
2. Background of the Relevant Art
It is well known that steps can be taken to remove lifetime reducing dopants (usually some of the heavy metals) from regions of the circuit where their presence would degrade performance. Not only does their presence reduce the minority carrier lifetime, but the dopants can also lessen mobility in MOS structures and increase the resistivity for both n- and p-type material. High minority carrier lifetime is desirable for many reasons, including minimizing reverse junction leakage, improving bipolar transistor gain, and increasing refresh time in dynamic MOS memories. In order to remove the lifetime reducing dopants from active regions of the circuit, techniques such as "gettering" and "denudation" are generally employed prior to or during semiconductor processing.
As defined herein, "semiconductor processing" or processing of the silicon substrate generally refers to the steps necessary to pattern, grow, implant and deposit various materials and ions upon and into the substrate to form a plurality of monolithic devices. Prior to processing of the silicon substrate, many manufacturers incorporate pre-process denuding and gettering steps. Pre-process denuding and gettering generally involves conditioning the unprocessed silicon wafer as it arrives from the vendor. Instead of pre-process denuding and gettering, other manufacturers may perform denuding and gettering (i.e., process-induced denuding and gettering) during the circuit processing steps.
As set forth in U.S. Pat. Nos. 4,666,532 and 4,548,654 (incorporated herein by reference), the term gettering generally refers to the precipitation of oxygen in substrate materials and the production of lattice strain, dislocation loops and stacking faults within the substrate bulk. The strain, loops and faults are often the result of SiO.sub.x precipitation growth. Precipitation growth causes lattice disorders and dislocations which act as gettering sites (traps) for unwanted, lifetime reducing dopants. Gettering can often be performed in inactive sites distant from the surface region of the silicon substrate and therefore is often referred to as intrinsic gettering. Intrinsic gettering not only produces the gettering sites resulting from precipitation, but also produces certain intrinsic material properties of Czochralski (CZ) grown silicon wafers (e.g., interstitial oxygen, substitutional carbon, silicon-vacancies and interstitial silicon).
The concept of producing gettering sites within the bulk silicon away from the active region is recognized in the art as providing substantial improvements in semiconductor wafer yield. However, the industry also recognizes the importance of depleting oxygen in the silicon substrate in the active regions near the substrate surface where devices are formed. Oxygen depletion in this region is generally referred to as denuding, and involves the out-diffusion of interstitial oxygen atoms from a wafer surface area (denuded area).
To improve device performance, both gettering and denudation steps are necessary. Conventional intrinsic gettering and denuding are generally performed as either a pre-process, three-step cycle or are incorporated (i.e., process-induced) within the processing steps of semiconductor fabrication. See, e.g., Borland, "Borland's Overview of the Latest in Intrinsic Gettering: Part I and Part II", Semiconductor International (April, 1989 and May, 1989) (incorporated herein by reference). The three-step intrinsic gettering, pre-process cycle generally involves subjecting the wafer as it arrives from the vendor to a high temperature ambient (usually greater than 1100.degree. C.) to out-diffuse the interstitial oxygen and form the denuded area directly below the substrate surface. Next, the wafer is cooled to a lower temperature (oftentimes less than 850.degree. C.) in order to allow nuclei of SiO.sub.x to form. The three-step process is completed by thereafter subjecting the wafer to a higher temperature in the range of approximately 900.degree. C. to 1000.degree. C. to form large precipitates from the SiO.sub.x nuclei. SiO.sub.x precipitation growth exceeding a critical size will form intrinsic lattice defects and thereby produce gettered areas. It is primarily the second and third steps of the three-step process which is responsible for intrinsic gettering, while the first step is used for denuding.
As taught in U.S. Pat. No. 4,548,654, use of a reducing ambient, such as hydrogen, during the denuding step results in acceleration of the out-diffusion of oxygen by avoiding formation of a barrier film at the substrate surface. Hydrogen combines with the out-diffused oxygen to form water which is carried from the wafer surface. While hydrogen appears to provide acceleration of the denuding process, conventional knowledge appears limited as to other advantages hydrogen may provide at the denuding step.
Maintaining hydrogen ambient for subsequent preprocessing steps, mainly gettering steps, can be expensive and time consuming. Moreover, after oxygen is out-diffused, hydrogen is generally not necessary to perform gettering. Thus, it would be advantageous to eliminate pre-process gettering and incorporate the gettering steps into the normal process flow of device fabrication. However, conventional processing steps are not easily adapted for incorporating the second and third (low temperature and subsequent high temperature) gettering steps. Generally speaking, if a wafer is pre-process denuded, then well regions which are thereafter formed require a high temperature drive-in step. The high temperature drive-in may not allow sufficient formation of SiO.sub.x nucleation required by the second step prior to the higher temperature third step. Thus, many manufacturers provide all three steps of denudation and gettering prior to semiconductor processing. Alternatively, manufacturers might incorporate all three steps into the semiconductor process flow. In the former, pre-process gettering adds to the steps and costs necessary to produce an integrated circuit. In the latter, process-induced denudation and gettering cannot achieve the advantages of hydrogen denudation unless the process flow is retrofitted with a hydrogen chamber.
The denuding step is performed at higher temperatures necessary to move oxygen along interstitial sites away from the silicon substrate surface. After the denuded area is formed, any future high temperature cycles may cause migration of oxygen and/or interstitial silicon atoms into the denuded area and near the substrate surface. If the denuded area receives substantial amounts of oxygen or interstitial silicon, the denuding area will be compromised and may provide gettering of heavy metal atoms at what was previously a denuded area. The dislocations caused by ingress of oxygen and interstitial silicon not only affects electrical characteristics in the active or channel regions due to the presence of metal atoms, but can also adversely affect structures grown upon the denuded area. Accordingly, it is important to maintain the quality of the denuded area during subsequent processing steps which undergo thermal cycling. It is certainly important to maintain denude quality prior to and during specific critical steps in the process flow.
It would be advantageous to provide a process flow in which denudation occurs as a pre-process step and that gettering occur during process. It would be further advantageous that hydrogen be used not only to accelerate out-gassing of oxygen, but also to provide enhanced silicon surface properties. It would be still further advantageous that the silicon surface properties be maintained during the initial stages of device processing, up to and including the formation of the gate oxide and the polysilicon layer over the gate oxide. The advantages of this methodology is to maintain the quality of the denuded area at least until after certain critical processing steps so that structures formed on the denuded silicon surface will have enhanced characteristics.